Yet Another Admin’s Blog

How often do you think about the reasons why your favorite RDBMS sucks? 🙂 Last few months I was doing this quite often and yes, my favorite RDBMS is MySQL. The reason why I was thinking so because one of my recent tasks at Scribd was fixing scalability problems in documents browsing.

The problem with browsing was pretty simple to describe and as hard to fix – we have large data set which consists of a few tables with many fields with really bad selectivity (flag fields like is_deleted, is_private, etc; file_type, language_id , category_id and others). As the result of this situation it becomes really hard (if possible at all) to display documents lists like “most popular 1-10 pages PDF documents in Italian language from the category “Business” (of course, non-deleted, non-private, etc). If you’ll try to create appropriate indexes for each possible filters combination, you’ll end up having tens or hundreds of indexes and every INSERT query in your tables will take ages.

So, we were doing many weird things to solve this problem, like having really huge covering indexes for most popular browsing situations, having powerful (and expensive) dedicated servers for browsing queries, etc. Nothing helped and we ended up reducing filters number to 1-2 per query. But with continuously growing documents base we’d really love to help people find interesting content and filters is what we’d really want to add to the project.

Long story short, we’ve decided to try to use Sphinx search engine for really unusual thing – we decided to move all SELECT queries for our documents to a small (proof of concept) sphinx installation on one of our DB servers and use Sphinx’ full-scan feature. After a few hours of development I’ve created a simple to use module which mimics ActiveRecord’s find method interface and allows us to perform queries like the following (sometimes even more powerful than AR):

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browser = Scribd::SphinxBrowser.new(:host=>'ip.add.re.ss')

# Get top 10 hottest docs
result = browser.browse

# Get top 10 most commented docs this week
result = browser.browse(:order=>'comments_week DESC')

# Get top 5 most voted docs of all times
result = browser.browse(:order=>'votes_total DESC', :limit=>5)

After a short testing we’ve got our performance benchmarks results and here are some facts:

documents count – 3000000+

re-indexing time – 20-30 seconds for an entire data set (4 large joined tables)

longest query (surprisingly – a query without any filters) takes about 0.5 sec on one core

generic query with 1-2 filters takes 0.01-0.05 sec on one core

queries with many filters take 0.01 sec and less on one core

queries time could be reduced almost linearly when adding more cores (splitting index on many pieces).

sphinx scales really easily on more than one box if we’ll need it to.

As you can see here, it is really useful sometimes to think “out of the box” when you work with some tools – we were using sphinx for a quite a long time now, but this solution for browsing was like a blessing and really helped us to keep up with service popularity growth.